(1) Field of the Invention
This invention relates to an apparatus and method for controlling the air to fuel ratio of an internal combustion engine.
(2) Prior Art
The air fuel ratio of an internal combustion engine can be determined by an analysis of the exhaust gases. For example, a sensing device located in the exhaust gas stream can sense the partial pressure of oxygen thereby determining whether the air fuel mixture is rich or lean of stoichiometry. Stoichiometry is the ratio of 14.6 parts air to 1 part fuel wherein complete combustion takes place. In many engine strategies, it is desirable to operate at stoichiometry.
A feedback control system in combination with the sensor can be used to vary the air fuel ratio supply to the internal combustion engine. The control system processes a signal derived from the sensing device and causes a change in the ratio in the air to fuel mixture applied to the internal combustion engine by, for example, increasing or decreasing the amount of fuel instantaneously added to a predetermined air quantity. The air fuel mixture ratio can be variable with engines having carburetors as well as fuel injection systems. Of course, the type of controller is advantageously adapted to the particular type of fuel supply systems.
A widely used technique to control the air fuel ratio in stoichiometric feedback controlled fuel metering systems is limit cycle integral control. In this technique, there is a constant movement of a fuel metering component in a direction that always tends to counter the instantaneous air fuel ratio indication given by a typical two state exhaust gas oxygen (EGO) sensor. For example, every time an EGO sensor indicates a switch from rich to lean air fuel ratio mode of operation, the direction of motion of a typical carburetor's metering rod reverses to create a richer air fuel ratio condition until the sensor indicates a change from a lean to rich air fuel ratio condition. Then, the direction of motion of the metering rod is reversed again this time to achieve a leaner air fuel ratio condition.
Referring to FIGS. 1a and 1b, step like changes in the sensor output voltage initiate ramp like changes in the actuator control voltage. When using the limit cycle or integral control, the desired air fuel ratio can only be attained on an average basis since the actual air fuel ratio is made to fluctuate in a controlled manner about the average value. The limit cycle system can be characterized as a two state controller and the mode of operation can be rich or lean. The average deviation from the desired value is a strong function of a parameter called engine transport delay time, .tau.. This is defined as the time it takes for a change in air fuel ratio, implemented at the fuel metering mechanism, to be recognized at the EGO sensor, after the change has taken place.
The engine transport delay time is a function of the fuel metering systems's design, engine speed and air flow. Because of this delay time, a control system using a limit cycle technique always varies the air fuel ratio about a mean value in a cyclical manner, for example, a richer fuel ratio typically followed by a lean air fuel ratio with overshoots. The shorter the transport delay time is, the higher will be the frequency of rich to lean and lean to rich air fuel ratio fluctuations and the smaller will be the amplitudes of the air fuel ratio overshoots. It can be appreciated that a system with no engine transport delay time is the ideal.
Known control devices for changing the air fuel ratio have various drawbacks. For example, the control apparatus may include a motor which operates a valve controlling the air fuel ratio, the motor having a preset driving speed. Because of the fixed driving speed changing the air fuel ratio, the desired change is not instantaneous and, during transition, the instantaneous air fuel ratio is different from the desired air fuel ratio. The engine transport delay time also causes a delay from the time of the change in the air fuel ratio at the intake system to the time the gas sensor senses the change at the exhaust system. This can produce unsatisfactory control of the air fuel ratio. If the delay time increases due to such conditions as low rotational speed, the control apparatus may be susceptible to a hunting phenomenon wherein the actual air fuel ratio has oscillatory values compared to the desired air fuel ratio. As is known, any variation from the desired air fuel ratio may reduce drivability, decrease mileage and deteriorate quality of the exhaust gas thereby increasing pollutants.
Known air fuel ratio controllers have a disadvantage in that the integration time constant for correction of the air fuel ratio is independent of engine speed. The main delay within the control loop (which includes the exhaust sensor, the controller, and the adjustment mechanism controlling the actual air fuel ratio mixture) is given by the time which the mixture takes on the path from the carburetor, or injection system, through the internal combustion engine. The air fuel mixture must pass through the internal combustion engine, and be delayed by the various strokes of the combustion engine, before the controller becomes sensitive to the exhaust gases and can determine the change in the composition of the exhaust gases. If an average, medium speed of the engine is assumed by picking an appropriate integration constant, then when the speed of the engine is low, the longer time of passage of the air fuel mixture through the engine causes integration of the integral controller to be too rapid. Correction of the mass ratio of the air fuel mixture applied to the internal combustion engine will thus be excessive and a deviation from command value in the opposite direction will result. Conversely, at speeds higher than the speed for which the integral controller operates at optimum value, the control effect is too slow, and the desired command value is reached only slowly. The control of the air fuel ratio should be accurate over the speed range of the engine. Further, the system and apparatus should be inexpensive and simply constructed. These are some of the problems this invention overcomes.